The Carbon Cycle Explained

by L.J. Martin

Most people have heard that we should all cut down on our carbon
emissions. This means we should release less carbon dioxide (CO2) into
the atmosphere. However, this is an overly simplistic definition. The
problem is not CO2 in itself, but CO2 from the burning of fossil fuels.

All living organisms, apart from some specialized bacteria, are
always releasing CO2 into the environment. Many people are confused
about this, and think that all CO2 emissions are bad. This is not true,
as CO2 emissions from living organisms are perfectly natural and
harmless, while CO2 emissions from fossil fuels may be said to be bad.
To understand this distinction, you must understand The Carbon Cycle.

The carbon cycle refers to the path of carbon atoms around the
biosphere, which includes the atmosphere and all living things. There
are an infinite number of paths around the carbon cycle. The carbon
always starts off and ends up in the atmosphere, which is the carbon
cycle's reservoir. There are many possible routes round the cycle. The
first step is photosynthesis, where CO2 is absorbed by plants or algae
and turned into glucose. The process by which living things burn
carbon and release CO2 is called respiration. The CO2 could also be released by physical burning,
for example in a forest fire.

Diagram of the Carbon Cycle

There are an infinite number of paths around the carbon cycle.
The carbon could be absorbed by a plant,
burned by that plant as fuel and returned to the atmosphere within
minutes or hours. Alternatively, it could be stored within a plant for
many months in fruits or grains, then eaten by an animal, which
burns the carbon for fuel and returns the CO2 into the atmosphere. A
plant or animal could die and be broken down by bacteria, which burn
the carbon for energy and return it to the atmosphere as CO2. Plants
could also be burned by humans for energy, returning the carbon to the
atmosphere as CO2. Once the carbon has completed the cycle, it will
start over again, and be endlessly recycled.

It is important to understand that all CO2 released by living things
has come from the atmosphere relatively recently, generally within the
past year or two. An exception is in the decay or burning of trees,
which have held onto their carbon for decades, or a couple of centuries
at most. This is still recent, in relative terms. The amount of CO2 in
the atmosphere decreases when a plant photosynthesizes, then increases
back to its initial level when that carbon is burned and returned back
into the atmosphere. In other words, the release of CO2 from living or
recently living organisms has no overall effect on atmospheric CO2
levels, and is said to be carbon neutral. When
natural materials are burned for energy, they are called biofuels. As
these are within the carbon cycle, and burning them simply carries on
the cycle, they are carbon neutral. Biofuels, including fuels such as
biodiesel, bioethanol and wood, can never cause an overall increase in
atmospheric CO2, as the carbon in biofuels recently came from the
atmosphere.

This is in stark contrast to fossil fuels. They were also part of
the carbon cycle, but instead of the carbon being released back into
the atmosphere, it was locked up beneath the earth's surface for tens
of millions of years. The level of CO2 in the atmosphere decreased when
this fossil carbon was locked up. When these fossil fuels are burned,
the atmospheric CO2 levels increase. There is not a short term balance
of CO2 reductions and increases as there is in the normal carbon cycle.
Fossil fuels, such as coal, natural gas and oil, increase the overall
level of carbon in the biosphere, and the atmosphere in particular.

To sum up, carbon is endlessly recycled through the biosphere in a process called the carbon cycle.
It exists in carbon dioxide gas, and in organic molecules such as fats,
oils, proteins and alcohols. CO2 breathed out by animals, or created
from burning plant materials, is balanced by the CO2 consumed by plants
at the start of the carbon cycle, and is considered carbon neutral.
Burning fossil fuels adds extra carbon to the cycle, as fossil carbon
has been locked away outside the cycle for millions of years. This is
why fossil fuels are considered bad by many people, while carbon
neutral biofuels are considered good.